GB2109274A - Gas turbine engine rotor assembly - Google Patents

Abstract

A method of assembling a gas turbine engine rotor comprises locating a bladed ring (10) on the periphery of a disc (13) and subjecting the assembly to hot isostatic pressing so as to achieve diffusion bonding across the interface between them. A ring-shaped plate member (15) is laid over each of the joint lines between the bladed ring (10) and the disc (13) and sealed in place with a high melting point sealant (18) prior to hot isostatic pressing. During hot isostatic pressing, diffusion bonding also occurs between the plate members (15) and the bladed ring (10) and disc (13). This diffusion bonding of the plate members (15) may be easily inspected by ultrasonic means and provides a ready indication of the effectiveness of the degree of diffusion bonding across the interface between the bladed ring (10) and the disc (13). <IMAGE>

Description

SPECIFICATION Gas turbine engine rotor assembly This invention relates to a gas turbine engine rotor assembly and in particularto a method of manufacturing a gas turbine engine rotor assembly which facilitates its inspection.

Rotor assemblies which are intended for use in gas turbine engines usually comprise a disc having an annular array of aerofoil blades equally spaced apart around its outer periphery. The aerofoil blades are usually attached to the disc by some form of mechanical connection. Thus it is common to provide each aerofoil blade with a fir-tree configuration root which locates in a correspondingly shaped cut-out portion in the disc periphery. Whilst such methods of attachment are satisfactory when employed in medium to large size gas turbine engines, they are not so successful when employed in small gas turbine engines. Because of the physical limitations in manufacturing small scale components, rotor blade aerofoil cross-sections are disproportionally large in comparison with the rotor disc to which they are required to be fitted.They therefore in turn require relatively large fir tree type root fixings to adequately retain them on the disc at high rotational speeds.

As speeds and temperatures are increased in orderto meet the demands of improved performance, then the proportions of fir tree type fixings need to be increased still further. Thus a mechanical limitation is reached with overcrowding of the blade root fixings on the rotor disc periphery. It is therefore desirable to provide an alternative method of attaching rotor blades to discs in which the use of mechanical root fixings is eliminated, thereby alleviating the problem of overcrowding on the disc periphery.

In UK Patent No 1586331 there is described a method of manufacturing a rotor assembly in which the disc is formed with a plane periphery and the aerofoil blades constitute part of a ring which is adapted to fit on that periphery. The joint lines between the ring and disc are brazed under vacuum after which the assembly is hot isostatically pressed to achieve diffusion bonding across the interface between the ring and the disc. Thus the method permits closer spacing of aerofoil blades around the periphery of a small diameter disc since there is no mechanical connection between the aerofoil blade and the disc.

One problem associated with constructing rotor assemblies by this method concerns the inspection of the diffusion bond between the bladed ring and the disc. Thus diffusion bonds of this type are conventionally inspected using ultrasonic techniques. Such techniques are most effective if the ultrasonic waves from the inspection apparatus are directed on to the diffusion bonded joint investigation in a direction which is substantially normal to the joint. However in the case of rotor assemblies, it is not possible to do this in view of the location of the diffusion bonded joint and the typical geometry of rotor assemblies. Generally it is only possible to direct ultrasonic waves on to the joint in a direction which is other than normal to the joint.This leads to poor attenuation of the reflected ultrasonic waves and a correspondingly poor indication of the integrity of the diffusion bonded joint.

It is an object of the present invention to provide a method of manufacturing a rotor assembly for a gas turbine engine by the diffusion bonding of a bladed ring to a disc wherein improved inspection of the diffusion bond is facilitated.

According to the present invention, a method of manufacturing a rotor assembly for a gas turbine engine comprises the steps of fabricating a metal bladed ring comprising a ring member having an annular array of radially extending aerofoil blades mounted around its outer periphery, preforming a metal disc, mounting said bladed ring on the outer periphery of said disc so that it is a close fit thereon and so that at least one annular joint line therebetween lies in a plane which includes adjacent portions of both said bladed ring and said disc, overlaying said at least one annular joint line with at least one metal plate member so that said at least one metal plate member is a close fit upon each of said bladed ring and said disc, providing a seal between the peripheral surface or surfaces of said at least one plate member and said bladed ring and said disc member under vacuum conditions with a high melting point sealant so as to fully enciose and seal the interface between said bladed ring and disc member, and subsequently hot isostatically pressing the resultant assembly to achieve diffusion bonding between all of the bladed ring, disc member and said at least one plate member.

Said at least one plate member is preferably machined off said resultant rotor assembly after said hot isostatic pressing.

Said at least one plate member is preferably provided with at least one groove, so positioned as to the adjacent one or both of said bladed ring and said disc member to prevent the ingress of said high melting point sealant into said interface between said bladed ring and said disc member and so dimensioned as to collapse during said hot isostatic pressing.

Said at least one plate member may be ringshaped.

Said bladed ring is preferably mounted on said disc member so that there are two annular joint lines between them, each of said annular joint lines being overlaid by one of said ring-shaped plate members.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a side view of a ring provided with an array of radially extending aerofoil blades.

Figure 2 is a side view of a disc.

Figure 3 is a side view of the bladed ring shown in Figure 1 mounted on the disc shown in Figure 2.

Figure 4 is a side view of the bladed ring/disc assembly shown in Figure 1 to which two ringshaped plate members have been attached to overlay each of the annular joint lines between the bladed ring and disc.

Figure 5 is a cross-sectional view of a portion of the bladed ring/disc assembly shown in Figure 4 showing the attachment of ring-shaped plate members to each of the annular joint lines between them.

With reference to Figure 1, a bladed ring generally indicated at 10 comprises a ring 11 which is provided with an annular array of equally spaced apart radially extending aerofoil blades 12 around its outer periphery. The bladed ring 10 may be fabricated by any convenient method. Thus, for instance, it may be fabricated by assembling a plurality of aerofoil blades having platforms in an annular array so that the platforms of adjacent blades abut and then bonding the abutting blades together by, for instance, electron beam welding. Alternatively, it may be fabricated byforming a ring having a plurality of aerofoil cross-section slots in its periphery, inserting aerofoil blades of corresponding cross-sectional shape in the slots and subsequently fixing the blades in the slots by electron beam welding.

It will be appreciated however that other methods of fabricating the bladed ring 10 could be employed if desired. After fabrication, the internal periphery of the ring 11 is machined to a given diameter.

The bladed ring 10 may be fabricated from any convenient alloy but we prefer to fabricate it from a nickel base alloy and in particular from the nickel base alloy known as MAR M002.

The disc 13 shown in Figure 2 is preformed from particles of an alloy which has been compacted under suitable conditions of heat and pressure so that the particles are consolidated. The alloy is also a nickel base alloy and is preferably that which is known as "Astralloy". The disc 13 is so formed that its outside diameter is slightly greater than the internal diameter of the bladed ring 10. The disc 13 is, however, machined after consolidation until its outside diameter is equal to the inside diameter of the bladed ring 10.

The bladed ring 10 is then heated up so that it thermally expands to such an extent that it may be mounted on the outer periphery of the disc 13 in the manner shown in Figure 3. The whole assembly of bladed ring 10 and disc 13 is then allowed to cool so that the ring 11 contracts on to the disc 13 so as to provide a close fit between them. Thus when the ring 11 is mounted upon the disc 13, two annular joint lines 14 are defined by them, one each side of the resultant bladed ring/disc assembly. The ring 11 and the disc 13 are so configured that each of the joint lines 14 lies in a plane which includes adjacent portions of both of ring 11 and the disc 13.

The joint lines 14 are each overlayed buy a ringshaped nickel base alloy plate member 15 can be seen in both Figures 4 and 5 so that each plate member 15 is a close fit upon each of the ring 11 and the disc 13. Each plate member 15 is provided with two annular grooves 16 and 17 which are so positioned that one groove is located each side of each joint line 14. The radially inner and outer peripheral surfaces of each plate member 15 are vacuum brazed to the ring 11 and disc 13 as indicated at 18 so that the interface between the ring 11 and the disc 13 is both enclosed and sealed. The annular grooves 16 and 17 may be upto 0.1 inches radius and act as braze traps so as to prevent the flow of molten braze material into the interface between the ring 11 and disc 13.It will be appreciated that other high melting point sealants could be used to provide a seal between the plate members 15 and the ring 11 and disc 13. Thus for instance electron beam welding could be employed in the sealing operation.

In an alternative method of assembly one of the plate members 15 is vacuum brazed on to the disc 13 before the ring 11 is mounted on the disc 13. Thus the plate member 15 acts as a location feature when the ring 11 is mounted on the disc 13 so as to ensure that the ring member 11 and disc 13 are aligned. The other mounting plate 15 is then appropriately mounted on the ring 11 and disc 13 and vacuum brazing is carried out as before.

After the plate members 15 have been sealed in position on the ring 11 and disc 13, the whole assembly is subjected to hot isostatic pressing. Thus the assembly is subjected to a pressure of 15000 pounds per square inch at a temperature of 1220 C for four hours. This serves to achieve diffusion bonding between the ring 11 and the disc 13 as well as between the plate members 15 and both the ring 11 and the disc 13. Moreover it causes the annular grooves 16 and 17 to collapse so that a continuous diffusion bond is created between the plate members 15, and the ring 11 and disc 13.

After hot isostatic pressing has been discontinued, the resultant rotor assembly is inspected by ultrasonic examination. Now since the diffusion band between the ring 11 and disc 13 is so positioned that it cannot be satisfactorily inspected ultrasonically, ultrasonic examination is made of the diffusion bonds between the plate 15 and the ring 11 and the disc 13. Thus the location of the plate members 15 is such that an ultrasonic probe may be positioned substantially normally to the diffusion bond under investigation. This provides effective examination of the diffusion bonds between the plates 15 and the ring 11 and disc 13. Thus we have found that if these diffusion bonds are satisfactory then so will be the diffusion bond between between the ring 11 and the disc 13.This is especially so in the case of the grooves 16 and 17 since if the hot isostatic pressing is sufficient to collapse them and provide diffusion bonding at their position of collapse, then diffusion bonding will also have occurred between the ring 11 and the disc 13. If a satisfactory degree of diffusion bonding has been detected, the plate members 15 are removed by machining them off.

In order to ensure that ultrasonic surface effects are avoided, it is usually necessary to ensure that the thickness of the plate members 15 is appropriate for the type of ultrasonic equipment which is to be used.

Thus the plate members 15 may be greater than 0.3 inches thick.

It will be appreciated that variations are possible within the general ambit of the present invention.

Thus although the present invention has been described with reference to plate members 15 which are in the form of rings, they could, in fact, be disc-shaped. This would, of course, mean that sealing would only be necessary around the circum ference of the disc. A further possible variation is the provision of a shoulder on the ring or disc to prevent relative axial movement between them prior to sealing. The provision of such a shoulder would, of course, mean that only one plate member would be required. The joint line between the shoulder and the ring 11 or the disc 13 would be so positioned as to preclude its being overlaid by a plate member. In such a situation, the joint line would have to be directly sealed by the high melting point sealant.

Claims (7)

1. A method of manfacturing a rotor assembly for a gas turbine engine comprising the steps of fabricating a metal bladed ring comprising a ring member having an annular array of radially extending aerofoil blades mounted around its outer periphery, preforming a metal disc, mounting said bladed ring on the outer periphery of said disc so that it is a close fit thereon and so that at least one annular joint line therebetween lies in a plane which includes adjacent portions of both said bladed ring and said disc, overlaying said at least one annular joint line with at least one metal plate member so that said at least one metal plate member is a close fit upon each of said bladed ring and said disc, providing a seal between the peripheral surfsace or surfaces of said at least one plate member and said bladed ring and disc member, and additiionally sealing any other exposed joint lines between said bladed ring and said disc member under vacuum conditions with a high melting point sealant so as to fully enclose and seal the interface between said bladed ring and disc member, and subsequently hot isostatically pressing the resultant assembly to achieve diffusion bonding between all of the bladed ring, disc member and at least one plate member.

2. A method of manufacturing a rotor assembly as claimed in claim 1 wherein said at least one plate member is machined off said resultant rotor assembly after said hot isostatic pressing.

3. A method of manufacturing a rotor assembly as claimed in claim 1 or claim 2 wherein said at least one plate member is provided with at least one groove so positioned as to be adjacent one or both of said bladed ring and said disc member to prevent the ingress of said high melting point sealant into said interface between said bladed ring and disc member and so dimensioned as to collapse during said hot isostatic pressing.

4. A method of manufacturing a rotor assembly as claimed in any one preceding claim wherein said at least one plate member is ring-shaped.

5. A method of manufacturing a rotor assembly as claimed in any one preceding claim wherein said bladed ring is mounted on said disc member so that there are two annular joint lines between them, each of said annular joint lines being overlaid by one of said ring-shaped plate members.

6. A method of manufacturing a rotor assembly as claimed in any one preceding claim wherein said high melting point sealant is a high melting point brazing alloy.

7. A method of manufacturing a rotor assembly substantially as hereinbefore described with reference to and as shown in the accompanying drawings.